The exhaust passage of a diesel engine is sometimes fitted with a lean NOx catalyst (referred to as LNC hereinafter) for reducing and eliminating nitrogen oxides (referred to as NOx hereinafter) in the exhaust gas, where the NOx is particularly generated in a large amount in lean combustion.
The LNC functions to trap (more specifically adsorb) NOx in an oxidizing atmosphere where an exhaust air fuel ratio is higher than a prescribed value (referred to as “lean” hereinafter) and reducing the trapped NOx into a harmless form in a reducing atmosphere where the exhaust air fuel ratio is lower than the prescribed value (referred to as “rich” hereinafter). The NOx purification ability of the LNC tends to decrease as the amount of trapped NOx increases. Therefore, in order to avoid saturation of amount of NOx trapped by the LNC, a process for regenerating the LNC is conducted by executing a rich spike control from time to time to make the exhaust air fuel ratio rich and reduce the NOx trapped by the LNC.
In the rich spike control, the reducing atmosphere is created in the exhaust system by decreasing an amount of air intake by restricting the opening of the intake control valve and/or increasing an amount of exhaust gas recirculation (EGR) than in a usual operation, and at the same time increasing an amount of fuel injection. It is common that the fuel injection amount is feedback-controlled such that an actual value of exhaust air fuel ratio detected by an O2 sensor or the like approaches a target value.
During the rich spike control, in order to shorten the time period from the start of control to the convergence of the exhaust air fuel ratio to the target value, it is conceivable to add, depending on an actual amount of air intake, a predetermined increment of fuel injection (feedforward term) to the current amount of fuel injection, in addition to an increment of fuel injection (feedback term) that is feedback-controlled based on a difference between the target and actual values of exhaust air fuel ratio. The actual amount of air intake may be measured by an air flow meter.
However, because characteristics of some component parts such as the air flow meter, fuel injection valve or the like may differ from one to another and also may change with time, the predetermined value of the feedforward term can be or become inappropriate, and this can result in unsatisfactory control accuracy and response characteristics, which in turn can lead to increased emission in the exhaust gas and lower fuel economy.
In order to cope with such problems and thereby improve the response characteristics in the rich spike control, Japanese Patent Application Laid-Open Publication No. 2002-201985 has proposed to conduct a stoichiometric (referred to as “stoic” hereinafter) combustion at a constant interval and learn a correction value used in the feedback control.
However, in the technique disclosed in JPA 2002-201985, it is necessary to conduct the stoic combustion only for the purpose of learning the correction value and this can deteriorate the fuel economy and/or drive characteristics. Further, the learning is not always possible and requires a certain time period of stationary driving in order to maintain desired control accuracy, and thus the opportunities for learning are inconveniently limited.